1. Field of the Invention
The present invention relates to a microwave antenna, particularly but not exclusively, to a constant E-plane beamwidth antenna.
2. Description of the Related Art
It is well known, for example from U.S. Pat. No. 4,667,205, that the width of a beam radiated by a horn antenna varies as a function of the wavelength and therefore as a function of the frequency. U.S. Pat. No. 4,667,205 discloses a wide band microwave antenna which in a given plane can cover a very wide angular field. The antenna comprises three parts: a rectangular cross section feeder which communicates with a first sectoral horn which is sectoral in the H-plane. The first sectoral horn communicates with a second sectoral horn having a partial cylindrical shape with circular-shaped outer edges. The second sectoral horn comprises top and bottom plates and a plurality of equally spaced, radially extending power distributors. The power distributors comprise metallic partitions extending in the H-plane between the top and bottom plates. The power distributors form a plurality of elementary radiation sources which distribute power across the face of a mouth curved in the second horn's E-plane. Optionally the first sectoral horn may be pyramidal.
The antenna constructed according to U.S. Pat. No. 4,667,205 has a number of drawbacks. One drawback is that the connection between the first and second sectoral horns is a sharp transition which may give rise to undesired reflections and to the generation of unwanted higher order modes. Since each mode propagates at a different speed which is frequency dependent then there will be some variation in the radiation pattern. A second drawback is that the theory behind such a horn is regarded as being very difficult so that it is envisaged that practical horns would be designed empirically by successive experimentation and modification.